Cellulose acetate tow cigarette filter tips sometimes soften during smoking of the cigarette thus reducing the filter tip effectiveness as a mouthpiece. This is especially true with the use of low denier per filament tow currently being used to achieve high filtration. This phenomenon is referred to as "filter collapse." Since the smoker thinks of the filter as the cigarette mouthpiece and is, therefore, most aware of the filter's rigidity, this collapse behavior creates an unpleasant sensation for the smoker because the "mouthpiece" then has a mushy feeling. This is considered extremely undesirable by most cigarette manufacturers. In extreme cases, as the filter tip softens, the tow partially separates from the wrap around the cellulose acetate tow, causing a reduction in the effectiveness of the filter tip for filtering smoke components.
In general, the factors that improve cold filter tip firmness also increase the resistance to collapse of the hot filter tip. However, it has been found that high levels of plasticizer application, which improve cold filter tip firmness, decrease the resistance to collapse for the hot filter tip.
In studies of collapse in which firmness is measured continuously on a filter tip as the cigarette is smoked, the results indicate that most softening occurs during the last two or three puffs, and the principle cause of filter collapse has been found to be the delivery of hot moisture to the filter tip.
In support of the last statement above, in a paper entitled "The Hardness of Cellulose Acetate Filters During Smoking" by J.N.T. White (presented at the CORESTA Symposium, November, 1982, and published by Courtaulds Acetate Limited, Cigarette Tow Division, Technical Services Dept., P.O. Box 13, Little Heath, Conventry, England), it was reported that temperature was not the only factor causing filters to soften. When hot air was drawn continuously through a filter so that the temperature within the filter element rose to the maximum found during smoking (about 80.degree. C.), very little softening occurred. If this hot air carried cigarette smoke with it, the softening increased and the effect was greater as the smoke concentration increased. The author made the conclusion that it appeared that a combination of smoke and temperature causes softening during smoking. The author expressed the thought that the contribution of the smoke may have a plasticizing effect on the fibers (of the tow making up the content of the filter) perhaps due to its water content. The author then said that the effect of water could be illustrated by drawing steamladen air through a filter instead of smoke-laden air. The author found, however, that the softening of the filter was extremely rapid and was much greater in extent than with smoke, although the temperature in the filter element did not rise above the normal smoking maximum.
Although the author of the above-entitled paper made no further comment about the use of steam-laden air, from my own experience I found that any attempt to use steam to simulate actual smoking through a filter element was ineffective and could not be correlated with actual smoking. Steam released under conditions of atmospheric pressure is laden with too much moisture, far more than what a filter tip would normally be subjected to by a smoker. Also such steam tends to be turbulent and uncontrollable, and subject to spurts and dripping with condensing water. The effectiveness of any filter tip exposed to this raw, uncontrolled steam is literally destroyed.
Collapse measurements, which are made in order to evaluate the collapse resistance imparted by different filter tow items and manufacturing techniques, are typically made with a transducer that converts the linear displacement of a weighted V-shaped foot in contract with a filter tip to a voltage. The weighted V-shaped foot engages the filter 5 millimeters behind the end of the filter tip where the tobacco column is joined. Care is taken to align the seam in the tipping paper away from where the weighted V-shaped foot is to engage the filter tip. The "tipping paper" serves to join the filter tip and the tobacco column together. As the weighted V-shaped foot gradually indents the hot and wetted filter tip of the cigarette being smoked, its displacement is recorded continuously. The displacement of the weighted V-shaped foot is observed at a specified length of unburned tobacco column to compute the percentage decrease in filter tip diameter, or in other words, the percentage of collapse.
A cigarette with its attached filter tip is smoked for collapse measurements by a machine, such as for example, by a single port syringe type smoking machine. The machine is adjusted to achieve standard smoking conditions of a two-second, 35 cubic centimeters (total), bell-shaped puff taken once every 60 seconds. In a "bell-shaped puff," the flow varies sinusoidally with time, going from zero flow at time zero increasing to a maximum flow after one second, and returning to zero flow after two seconds.
Before cigarettes are tested, they are conditioned at 22.degree. C. and at 60% relative humidity for at least 24 hours prior to testing. Testing should also be carried out in an environment controlled to these values. A mark is made on the cork tipping of each cigarette at a point 5 millimeters from the junction of the tobacco column and the filter tip measured in the direction of the mouthpiece end. The mark should be made at 90 degrees to the lap seal. The cigarette is also marked at its standard butt length, i.e., the point to which the cigarette will be smoked for test purposes.
The smoking apparatus is turned on and allowed to warm up prior to its being calibrated. Calibration and linearity of response are then evaluated by inserting circumference standards under the weighted V-shaped foot and recording the output signal of the linear variable differential transformer or transducer (LVDT). Calibration is accomplished, for example, by zeroing the recorder using a standard that is 7.90 millimeters in diameter and adjusting the full scale recorder deflection using a 6.30 millimeters standard. Full scale deflection based upon the calibration corresponds to a collapse of 20% which should be adequate for measuring just about any cellulose acetate filter. If the operator has prior information regarding the expected range of collapse for a set of samples, then the calibration procedure can be modified to give optimum response by choosing standards which just bracket the desired range. Linearity is checked by measuring several standards of intermediate diameter and plotting the output signal of the LVDT versus diameter.
A filter cigarette is inserted into the labyrinth seal of the smoking machine and positioned under the weighted V-shaped foot of the LVDT such that the foot rests on the mark previously made on the filter tip. The recorder is then started, a 15 gram weight is placed on the loading pan of the V-shaped foot, the smoking machine is switched on and the cigarette is lit. The cigarette is smoked to the standard butt length as previously marked and is then extinguished.
The percent filter collapse during smoking is calculated from the following equation: EQU % collapse=100.times.(b/a)
wherein "b" is the depth to which the weighted V-shaped foot penetrated into the filter tip after the last puff and "a" is the filter tip diameter or position of the weighted V-shaped foot prior to lighting the cigarette but after applying the weight. Although the percent collapse is calculated for the last puff taken up to the butt mark, for some applications it may be necessary to observe the collapse profile during smoking in order to describe this behavior as a function of puff number being drawn.
It takes approximately ten (10) minutes to smoke a cigarette by machine and about two to three minutes to prepare another cigarette for smoking. This limits the number of cigarettes that can be tested and analyzed during the course of a normal working day. Consider also the 24 hour conditioning period required before any test may be started.
An object of the invention, therefore, is to reduce significantly the time required to test a filter tip for its resistance against collapse.
Another object of the invention is to improve precision of measurement and separate filter performance from tobacco combustion variables while maintaining high correlation with collapse measured during smoking.